When two granular phases are brought into motion in a rotating drum, a competition of mixing and segregation occurs. Several image analysis methods have been used to quantify the mixing. In this work, a modification of the contact method, originally proposed by Van Puyvelde et al. (Powder Technol. 106, 183–191 (1999)), is suggested to allow evaluation of the mixing index for each separate image. A strength of this modified method lies in the removal of the case-dependent normalization of the mixing index, which has so far impaired a direct comparison to other studies. This modified method is tested on artificial and experimental images of a granular bed composed of spherical glass and polypropylene beads of equal size. The bed evolves in a rotating drum under the rolling regime. The temporal evolution of the mixing index is in excellent agreement with the commonly used variance method.